This invention relates generally to RF energy sources and more particularly to cross-field type RF energy sources.
As is known in the art, magnetron devices, such as magnetron oscillators, cross-field amplifiers, and the like, are used in radio frequency systems such as radar systems to provide a source of RF energy in the microwave/millimeter wave frequency range. Magnetron based devices convert DC power into relatively high power RF signals with relatively high conversion efficiency.
As is also known, a magnetron includes an evacuated enclosure and an electrode, commonly referred to as a cathode, which is heated to provide thermionically emitted electrons having sufficient mobility to provide a current flow between the heated cathode and at least one additional electrode generally referred to as the anode. The magnetron converts DC power by the interaction of electrons with the electric field of a circuit element in crossed DC electric and magnetic fields. During the interaction process, electron potential energy is converted into the RF energy of the electromagnetic fields resulting in high power signals at microwave and millimeter wave frequencies.
As is also known in the art, there exists a trend toward providing lower noise levels in microwave and millimeter wave radar systems. One problem with magnetron devices used in such systems is that these devices convert DC power to RF power having relatively high noise levels which degrade the sensitivity of radar systems using such devices.
As is also known in the art, a magnetron oscillator is provided by combining a cavity resonator with a magnetron. One example of a magnetron oscillator is the so-called coaxial magnetron oscillator. The coaxial magnetron oscillator includes an anode and a cathode. The anode is provided as a cylindrical resonant cavity of a conductive material such as copper. A portion of the anode provides a resonant circuit which in combination with the resonant cavity determines the frequency of operation of the oscillator. The resonant circuit may be provided by a plurality of electrically conductive rectangular blocks, often referred to as vanes, with each vane having one end terminated at the cylindrical resonant cavity and having a second end extending inward toward the center of the cavity. The vanes function as resonant circuits and serve a purpose similar to that of the lumped constant LC resonant circuits used at lower frequencies. The cathode may be provided as a cylinder of oxide-coated material disposed at the center of the anode.
The region between the anode and the cathode is called the interaction space. The interaction space is the region in which electrons from the cathode interact with DC electric and magnetic fields and the RF electric field in such a manner that the electrons impart their energy to the RF field provided in the interaction space.
Another type of coaxial magnetron is the so-called inverted coaxial magnetron. In this magnetron, the cathode surrounds the anode. A stabilizing TE.sub.011 cavity is in the center of the magnetron with the so-called vane type resonator system, previously described, arranged on the outside of the cavity. The cathode is provided as a ring surrounding the anode. Power is coupled from the end of the central stabilizing cavity by a waveguide.
Another device which uses the general magnetron concept is a cross-field amplifier (CFA). The CFA includes input and output ports, an anode slow-wave circuit and a cathode. An RF drive signal is provided at the input of the anode slow-wave circuit and a relatively high power RF output signal is provided at the output of the anode slow-wave circuit.
In one embodiment, electrons originate from a cylindrical cathode which is coaxial to the slow-wave circuit that acts as the anode. As in the magnetron oscillator, the region between the anode and the cathode is called the interaction space.
One technique which has been suggested to provide a low noise CFA is described in U.S. Pat. No. 4,928,070 by MacMaster et al. and assigned to the assignee of the present invention.
In the above mentioned patent low noise operation of a cross-field amplifier is provided by feeding an input RF signal to both a cathode slow-wave circuit and an anode slow-wave circuit. The relative amplitude and phase of each signal applied to the slow-wave circuits of the anode and the cathode are controlled, and the outputs of the anode and the cathode slow-wave circuits are terminated in matched characteristic impedances. This approach provides cross-field amplifiers with improved signal to noise ratios.
In both the magnetron oscillator and the cross-field amplifier, the electron and RF signal interactions which take place in the interaction space is similar. In the interaction space, primary electrons emitted from the cathode which are out of phase with the RF electric field interact with the RF electric field as well as the DC electric and magnetic fields in such a manner that the electrons give up their energy to the RF field. The magnetic field, which is axially aligned with the cathode, passes through the interaction space parallel to the cathode and perpendicular to the DC electric field.
The RF electric field and the crossed electric and magnetic fields cause the electrons to be completely bunched almost as soon as they are emitted from the cathode. After becoming bunched, the electrons move along in a traveling-wave RF electric field. This traveling-wave field moves at almost the same speed as the electrons, causing RF power to be delivered to the travelling wave.
Electrons emitted from the cathode in phase with the RF electric field are accelerated by the RF field. This acceleration causes the electrons to return to the cathode and strike the cathode. Such electron interaction is called back-bombardment. Back-bombardment increases the cathode temperature. An increase in the cathode temperature causes an increase in thermionic electron emission. Back-bombarding electrons also cause secondary electron emissions to be emitted from the cathode. Temperature increases due to back-bombardment are generally seen as reducing the operational lifetime of the cathode.
The secondary emission of the magnetron cathode surface may be reduced through the addition of slots provided in the cathode. Thus, cut away cathode surfaces have been used to reduce the number of secondary electron emissions from the cathode.
Nevertheless, in general, relatively high noise levels are characteristic of most magnetron type devices. For the reasons mentioned above, this is undesirable. It would be desirable to develop a technique applicable to both cross-field amplifiers and magnetron oscillators to reduce their noise levels to more acceptable levels.